MIMO
Many studies have been made to increase frequency efficiency to increase the speed of wireless data communication within a limited frequency band. Among them, MIMO (Multi-Input Multi-Output) technique to increase transmission capacity per unit frequency with a plurality of antennas used concurrently gains attention.
LP MU-MIMO
MIMO includes Single-User MIMO (SU-MIMO) in which a base-station (BS) apparatus transmits a plurality of signals to a single mobile-station (MS) device at the same timing and on the same frequency, and Multi-User MIMO (MU-MIMO) in which a base station transmits a signal to different mobile-station devices at the same timing and on the same frequency.
Since SU-MIMO is unable to multiplex streams more than the number of antennas of a mobile-station device, the maximum number of streams is limited by the number of physical antennas of the mobile-station device. On the other hand, since a base-station apparatus is able to have antennas more than the number of antennas of the mobile-station device, MU-MIMO becomes necessary in order to make the most use of idling antennas of the base-station apparatus. Specifications of down-link (DL) MU-MIMO using linear precoding (LP) have been formulated in LTE (long Term Evolution) and LTE-Advanced (see Non Patent Literature 1 below).
NLP MU-MIMO
In MU-MIMO based on LP (LP MU-MIMO), a base-station apparatus orthogonalizes transmit signals by performing a multiplexing operation on a linear filter, and thus removes Multi-User Interference (MUI) between mobile-station devices. This reduces a flexibility of combinations of mobile-station devices that can be spatial multiplexed.
On the other hand, Nonlinear Precoding (NLP) MU-MIMO is disclosed as another method to implement spatial multiplexing. In NLP MU-MIMO, a mobile-station device performs a modulo operation to treat, as the same point points, points to which a received signal is shifted in parallel by an integer multiple of a constant width (modulo width) in directions of an in-phase channel (I-ch) and a quadrature channel (Q-ch). In this way, the base-station apparatus can add to a modulation signal a signal of any integer multiple of the modulo width (perturbation vector), and reduces transmission power by appropriately selecting the perturbation vector and adding the selected perturbation vector to a signal addressed to each mobile-station device (see Non Patent Literature 2 below).
VP MU-MIMO
The mobile-station device performs the modulo operation on a received signal, and the base-station apparatus will have a freedom of adding to each modulation signal a signal of any integer multiple of the modulo width. The signal that can be added is referred to as a perturbation vector. VP (Vector Perturbation) MU-MIMO is a method of searching for a perturbation vector that increases power efficiency most, in view of channel states of all mobile-station devices that are spatial multiplexed. In VP MU-MIMO, the base-station apparatus has a large amount of calculation, but VP MU-MIMO is NLP MU-MIMO scheme that provides excellent characteristics with a full transmit diversity gain (see Non Patent Literature 2 below).
THP MU-MIMO
THP (Tomlinson-Harashima precoding) MU-MIMO is available and is a method different from VP MU-MIMO. THP MU-MIMO calculates a perturbation vector that is to be successively added to the signal addressed to each mobile-station device, in view of user interference each mobile-station device has suffered. In THP MU-MIMO, the complexity of a transmission process of the base-station apparatus is low but not all mobile-station devices may obtain full transmit diversity (see Non Patent Literature 3 below).
LR-THP
LR-THP is a method of THP MU-MIMO with a process called lattice reduction (LR) added thereto. LR-THP is a method that provides the full transmit diversity gain with an amount of calculation lower than the amount of calculation of VP MU-MIMO (see Non Patent Literature 3 below).
DMRS
In the NLP MU-MIMO system, a base-station apparatus needs to transmit DMRS (DeModulation Reference Signal) to each mobile-station device. However, if the base-station apparatus performs the same non-linear precoding operation as the data signal on DMRS and then transmits the DMRS, the mobile-station device is unable to estimate channels.
The DMRS is a signal that the base-station apparatus uses to notify each mobile-station device in advance of an amplitude and phase of the data signal on which the base-station apparatus has performed the precoding operation through NLP MU-MIMO. If a non-linear precoding operation is performed on the DMRS, the base-station apparatus adds a perturbation vector on the DMRS (or performs the modulo operation on the DMRS), and then transmits the DMRS. The mobile-station device needs to perform the modulo operation on the DMRS as well. For this reason, the mobile-station device needs to know a modulo width needed in the modulo operation in advance. Since the modulo width is proportional to the amplitude of the modulation signal in a received signal, the mobile-station device needs to know a reception gain of the data signal (a complex gain of a channel) subsequent to the non-linear precoding operation. However, the mobile-station device is unable to know the reception gain (the complex gain of the channel) without estimating channels using the DMRS. More specifically, the mobile-station device is in the situation that “the mobile-station device is unable to acquire the reception gain without estimating the channels using the DMRS, while being unable to estimate the channels using DMRS without knowing that reception gain”. The above-described problem thus arises.
In the technique disclosed in Patent Literature 1, the DMRS is transmitted to each mobile-station device using orthogonal radio resources (regions divided in a time direction and in a frequency direction, and if different data signals and different reference signals are assigned to the regions, the regions do not mutually interfere with). In such a case, the mobile-station device is free from performing the modulo operation on the DMRS, and obtains the reception gain.